Magnetron cathode sputter deposition devices perform deposition of coatings to substrates by ejection of atoms or groups of atoms from the surface of a cathode in a vacuum by heavy-ion impact on the substrate to be coated with the material of the cathode. A magnet array is rotated over the substrate to create a dynamic pattern of magnetic flux over the substrate surface area covered by the array. The pattern of the magnet array is critical to obtaining a uniform deposition thickness of material upon the substrate. The substrate is disposed in a low pressure vacuum into which an inert gas is introduced. A reactive gas may be introduced into the chamber for reactive ion etching. An RF voltage is applied to the substrate holder to create an RF field between the substrate and the chamber. Electrons are removed from the substrate and ionize the gas. Gas ions impact the substrate dislodging atoms or molecules of the substrate material. The ions are generally produced by collisions of gas atoms and electrons in a glow discharge and are accelerated into the target substrate from the cathode with the aid of the electric field. The substrates, arranged in the vacuum chamber, catch the atoms knocked out of the target material and in this way are coated with the cathode material. Uniform etching can also be accomplished by use of a rotating magnet array in a similarly constructed etching device.
A fundamental performance deficiency of magnetron cathodes of sputtering devices is that the power dissipation capacity is limited by the geometry of the cathode supporting flange and heat generated at the cathode/flange interface. In general, increasing the electric field strength increases the material deposition rate on the substrate, thus reducing cycle time. Increasing the field strength also increases the heat flux power density at the cathode. Several variations to magnetron cathode configurations have been proposed to increase the power density of the cathode. For example, welding the cathode to the flange interface allows the cathode to withstand higher temperatures. Alternatively, the flange and cathode can be machined from a single piece of cathode material. This type of construction however is expensive and is not reworkable by addition of new cathode material. Another approach is to increase the flow of chilled water used as a coolant within the magnet array housing. This approach has had limited effect on power dissipation in practice. Higher water pressure inside the magnet array housing results in increased mechanical stress on the cathode material solder joint, offsetting the benefit of increased cooling action of the water.
The cathode material in the vacuum chamber is typically either in the form of a single piece housing around the magnet array, or attached by weld or solder to a mounting surface of a permanent housing around the magnet array which seals the magnet array from the vacuum chamber. When the cathode material of a single piece housing is expired, the entire housing assembly must be replaced. Because the entire housing must be constructed of ultra-pure cathode material, replacement of the housing is very expensive.
In a different cathode design wherein the cathode material is attached to a permanent housing, the cathode is reworked by welding or soldering on new cathode material. The bonding interface between the housing and the cathode material is typically a solder bond of, for example, Pb, In, Sn or Ag selected for adhesive and heat transfer properties. However, when this method of cathode attachment is used with relatively large wafers having power level densities on the order 200 watts/in.sup.2, the interface material experiences delamination failures due to insufficient heat transfer. The larger surface area of large wafers creates excessive mechanical stress on the cathode plate/housing interface due to the presence of vacuum pressure on the target side and coolant pressure (typically on the order of 35-40 psi) on the inside of the housing.
The form of bonding of the cathode material to the housing must be able to withstand the heat generated in the ionization process. The cathode material and the bonding layer of the cathode to the housing are exposed to high temperatures, strong magnetic and electric fields and a high vacuum during sputtering. For this reason the interior of the housing and the magnet array is cooled by flooding with a fluid coolant, usually water.
The average maximum power density at the surface of the cathode presently is on the order of 100 watts/square inch. Increasing the power in permanent housing type cathodes can result in failure of the cathode bonding solder joint due to high temperatures and high thermal and mechanical stress. In order to achieve more efficient cooling, a higher velocity and flow rate of cooling fluid is required on the interior surfaces of the housing. Appropriate coolant circulation pressure is necessary. However, excessive coolant circulation pressure results in a relatively large difference between the pressure in the cooling chamber and the pressure in the vacuum chamber surrounding it, producing target distortion or ruptures in the target material or solder joint.